fix(lgrutil): child delr/delc not correct if parent has variable row/col spacings

[langevin-usgs]

• Fix child delr/delc calculations
• Add better Lgr access to parent and child grid information

[codecov]

Codecov Report

Merging #1403 (f092617) into develop (85d4558) will decrease coverage by 45.908%.
The diff coverage is 26.666%.

@@              Coverage Diff               @@
##           develop     #1403        +/-   ##
==============================================
- Coverage   75.948%   30.040%   -45.909%     
==============================================
  Files          249       249                
  Lines        52188     52123        -65     
==============================================
- Hits         39636     15658     -23978     
- Misses       12552     36465     +23913     

Impacted Files	Coverage Δ
flopy/utils/lgrutil.py	83.211% <26.666%> (-15.923%)	⬇️
flopy/mf6/utils/testutils.py	0.000% <0.000%> (-93.334%)	⬇️
flopy/mfusg/mfusgsms.py	6.190% <0.000%> (-91.474%)	⬇️
flopy/mf6/utils/postprocessing.py	9.259% <0.000%> (-88.889%)	⬇️
flopy/modflow/mfevt.py	7.812% <0.000%> (-87.007%)	⬇️
flopy/mf6/utils/lakpak_utils.py	2.097% <0.000%> (-85.315%)	⬇️
flopy/modflow/mfhyd.py	13.223% <0.000%> (-85.124%)	⬇️
flopy/utils/swroutputfile.py	10.714% <0.000%> (-85.079%)	⬇️
flopy/utils/mtlistfile.py	4.620% <0.000%> (-84.700%)	⬇️
flopy/modflowlgr/mflgr.py	6.271% <0.000%> (-84.354%)	⬇️
... and 142 more